Name: Period: Score: Everything About Chemical Formulas

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1 Name: Period: Score: Everything About Formulas Compounds have unique names that identify them for us when we study chemical properties and changes. Chemists have devised a shorthand way of representing chemical names that provides important information about the substance. This shorthand representation for a compounds name is called a chemical formula. You will practice writing chemical formulas in the following activity. WHAT IS A CHEMICAL FORMULA? formulas have two important parts: chemical symbols for the elements in the compound and subscripts that tell how many atoms of each element are needed to form the compound. The chemical formula for water, H 2 O, tells us that a water molecule is made of the elements Hydrogen (H) and Oxygen (O) and it takes 2 atoms of Hydrogen and one atom of Oxygen to build the molecule. The formula for a compound indicates the elements that make up the compound and the number of atoms of each element present in the compound. The number of atoms is indicated by the use of small numbers called subscripts. COUNTING ATOMS In the table, list each element in the compound and the number of atoms of each element present. Name Formula Atoms in Formula Calcium Carbonate CaCO 3 Ca = Calcium 1 C = Carbon 1 O = Oxygen 3 Aspirin C 9 H 8 O 4 Acetic Acid C 2 H 4 O 2 Pyrite FeS 2 Asbestos H 4 Mg 3 Si 2 O 9 Iron Oxide Fe 2 O 3

2 OXIDATION NUMBERS An elements oxidation number indicates how many electrons are lost or gained when chemical bonding occurs. The oxidation number is equal to the charge an atom has when it ionizes, that is, gains or loses electrons to become and ion. The partial periodic table below shows the most common oxidation numbers of the elements. The oxidation numbers are written above the group number above each column on the table. 1. Copy the oxidation numbers on the table shown above onto your periodic table. Place a box around each number (Example: above group 1 write 1+ ) On your key, indicate that any boxed numbers represent the oxidation number of that group. (Example for key: # = oxidation numbers) 2. What is the oxidation number of Calcium? 3. What is the oxidation number of Oxygen? 4. Why do elements in group 2 have an oxidation number of 2+? 5. Why do elements in group 17 have an oxidation number of 1-? 2

3 HOW TO WRITE A CHEMICAL FORMULA How do chemists know how many atoms of each element are needed to build a molecule? For ionic compounds, oxidation numbers are the key. An elements oxidation number is the number of electrons it will gain or lose in a chemical reaction. We can use the periodic table to find the oxidation number for an element. When we add up the oxidation numbers of the elements in an ionic compound, the sum must be zero. Therefore, we need to find the balance of negative and positive ions in the compound for the molecule to form. Example 1: A compound is formed by the reaction between magnesium and chlorine. What is the chemical formula for this compound? From the periodic table, we can find that the oxidation number of magnesium is 2+. Magnesium loses 2 electrons in chemical reactions. The oxidation number for chlorine is 1-. Chlorine tends to gain one electron in a chemical reaction. Remember that the sum of the oxidation numbers of the elements in a molecule will equal zero. This compound requires one atom of Magnesium with an oxidation number of 2+ to combine with two atoms of Chlorine, each with an oxidation number of 1-, for the sum of the oxidation numbers to be zero. (2+) + 2(1-) = 0 To write the chemical formula for this compound, first write the chemical symbol for the positive ion (Mg) and then the chemical symbol for the negative ion (Cl). Next, use subscripts to show how many atoms of each element are required to form the molecule. When one atom of an element is required, no subscript is used. The correct chemical formula for Magnesium Chloride is MgCl 2. Example 2: Aluminum and Bromine combine to form a compound. What is the chemical formula for the compound they form? From the periodic table, we find that the oxidation number for Aluminum (Al) is 3+. The oxidation number for Bromine (Br) is 1-. In order for the oxidation numbers of this compound to add up to zero, one atom of Aluminum must combine with three atoms of Bromine: (3+) + 3(1-) = 0 The correct chemical formula for this compound, Aluminum Bromide is AlBr 3. Short Cut: If you are struggling to determine the required number of atoms using math, try the criss-cross method instead. The criss-cross method is a simple way to determine the chemical formula of a compound. By criss crossing the oxidation numbers of each element, you quickly identify the required number of atoms in a compound. 3

4 PREDICTING CHEMICAL FORMULAS Compounds that are formed from ions are called ionic compounds. Predict the chemical formulas for ionic compounds that are made up of the pairs of elements in the table below. Use the following steps: 1. Using the periodic table, determine the symbol for each element. 2. Using the periodic table, determine the ion formed by each element. 3. Figure out how many of each element you would need to make a compound that is electrically neutral. 4. Draw the electron dot diagram to represent the compound 5. Write the correct chemical formula based on your combinations. (Hint: Make sure the Cation is written first and the Anion is written second.) Element Symbol (Step 1) Ion Formed (Step 2) Number of Atoms Needed (Step 3) Draw Dot Diagram (Step 4) Formula (Step 5) Calcium Ca 2+ 1 Bromine Br 1-2 CaBr 2 Magnesium Sulfur Hydrogen Fluorine Aluminum Oxygen Potassium Chlorine Lithium Sulfur 4

5 PRACTICE WRITING CHEMICAL FORMULAS FOR IONIC COMPOUNDS Use the periodic table to find the oxidation numbers of each element. Then write the correct chemical formula for the compound formed by the following elements: Element Symbol Oxidation Number Number of Atoms Needed Cation/Anion Formula Beryllium Be 2+ 1 Cation BeI 2 Iodine I 1-2 Anion Chlorine Calcium Sodium Oxygen Phosphorus Boron Sodium Bromine Beryllium Nitrogen 5

6 PRACTICE WRITING CHEMICAL FORMULAS FOR COVALENT COMPOUNDS Use the periodic table to find the oxidation numbers of each element. Then write the correct chemical formula for the compound formed by the following elements: Element Symbol Oxidation Number Number of Atoms Needed Polar or Non- Polar? Formula Hydrogen H 1+ 4 Polar CH 4 Carbon C 4-1 Iodine Chlorine Hydrogen Oxygen Sulfur Chlorine Fluorine Fluorine Silicon Hydrogen 6

7 POLYATOMIC IONS Have you ever heard of Sodium Nitrate? It is a preservative used in foods like hot dogs. The chemical formula for Sodium Nitrate is NaNO 3. How many types of atoms does this compound contain? You are correct is you said three: sodium, nitrogen and oxygen. The nitrogen and oxygen atoms have a shared electron bond. They act as one unit (called Nitrate) with an oxidation number of 1-. Ions that have more than one type of atom (like nitrate) are called polyatomic ions. To write the chemical formula for a compound containing one or more polyatomic ions, look at the table below to determine the ion s oxidation number. Then, use the same procedure for writing chemical formulas that you have practiced previously. Example: Calcium and the hydroxide ion (-OH) combine to form a compound. Write the chemical formula for this compound. From the periodic table, we see that the oxidation number for calcium is 2+. From the table above, you can see that the oxidation number for the hydroxide ion is 1-. To make a molecule of calcium hydroxide, we need one calcium atom and two hydroxide ions. (2+) + 2(1-) = 0 The correct chemical formula for this compound would be Ca(OH) 2. (Note that we enclose the members of the polyatomic ion in parentheses. The subscript for this ion is placed outside of the parentheses. This shows that we need two complete polyatomic hydroxide ions to form the compound.) 7

8 WRITING CHEMICAL FORMULAS FOR COMPOUNDS WITH POLYATOMIC IONS Use the table on the previous page and the periodic table to find the oxidation numbers of each ion. Then write the correct chemical formula for the compounds formed by these ions. Hint: DO NOT FORGET TO USE PARENTHESIS IF YOU NEED MORE THAN ONE POLYATOMIC IONS Element /Ion Symbol(s) Oxidation Number Number of Atoms/Ions Required Formula Sodium Na 1+ 3 Phosphate PO Na 3 PO 4 Calcium Nitrate Fluorine Ammonium Boron Sulfate Lithium Hydroxide Beryllium Carbonate Nitrogen Hydronium 8

9 CALCULATING FORMULA MASS A chemical formula gives you useful information about a compound. First, it tells you which types of atoms and how many of each are present in a compound. Second, it lets you know which types of ions are present in a compound. Finally, it allows you to determine the mass of one molecule of a compound, relative to the mass of other compounds. We call this the formula mass. In this section you will learn how to calculate the formula mass of a compound. A common ingredient in most toothpaste is a compound called sodium phosphate. If you examine a tube of toothpaste, you will find that it is usually listed as trisodium phosphate. What is the formula mass of trisodium phosphate? Step 1: Determine the formulas and oxidation numbers of the ions in the compound. Sodium phosphate is made up of the sodium ion and the phosphate ion. The oxidation number for the sodium ion can be determined from the periodic table. Since sodium, Na, is located in group 1 of the periodic table, it has an oxidation number of 1+ like all of the elements in group 1. The chemical formula and oxidation number of sodium is Na 1+ To find the formula and oxidation number for the phosphate ion, use the polyatomic ion chart in this packet. The chemical formula and oxidation number for the phosphate ion is PO 4 3- Step 2: Write the chemical formula of the compound. Remember that compounds must be neutral, the oxidation number of the elements and the ions must be equal to zero. Since Sodium = Na 1+ and Phosphate = PO 4 3- how many of each do you need to make a neutral compound? You need three sodium ions for each phosphate ion to make a neutral compound. The chemical formula of sodium phosphate is Na 3 PO 4 Step 3: List the atoms, number of each atom, atomic mass of each atom and total mass of each atom. Atom Number Atomic Mass (from the periodic table) Total Mass (number x atomic mass) Na amu 3 x = amu P amu 1 x = amu O amu 4 x = amu Step 4: Add up the values and calculate the formula mass of the compound amu amu amu = amu The formula mass of sodium phosphate is amu! 9

10 PRACTICE CALCULATING FORMULA MASS Using the previous page as a template, calculate the formula mass for the following compounds. 1. Calcium Phosphate What Ions are in this compound? What are the oxidation numbers of each ion? What is the Formula? ION #1 ION #2 Atom Number Atomic Mass (from the periodic table) Total Mass (number x atomic mass) Total Formula Mass 2. Barium Chloride What Ions are in this compound? What are the oxidation numbers of each ion? What is the Formula? ION #1 ION #2 Atom Number Atomic Mass (from the periodic table) Total Mass (number x atomic mass) Total Formula Mass 10

11 3. Magnesium Hydroxide What Ions are in this compound? What are the oxidation numbers of each ion? What is the Formula? ION #1 ION #2 Atom Number Atomic Mass (from the periodic table) Total Mass (number x atomic mass) Total Formula Mass 4. Ammonium Nitrate What Ions are in this compound? What are the oxidation numbers of each ion? What is the Formula? ION #1 ION #2 Atom Number Atomic Mass (from the periodic table) Total Mass (number x atomic mass) Total Formula Mass 11